The invention relates to electrical apparatuses for converting the relative motion of an object into an electrical signal and more particularly to an improved inductive transducer for measuring rotary displacement.
In the field, it is often necessary to convert a position of an object relative to an origin into an electrical signal. This output signal, which can be analog or digital is often then fed into other electrical devices. In process monitoring systems, for example, the signal can be inputted into an electrical indicator which outputs a display of the required data. Such data may include liquid pressure, linear position, tension, or any other like measurement. In process control systems, for example, the electrical signal can be used as feedback and inputted into the controller which acts, based on the nature of the signal, to control the position of the object relative to an origin.
Applications exist for inductive based measurements of linear displacement of a probe. In a device such as that disclosed by U.S. Pat. No. 4,667,158 to Redlich, the linear displacement of a metallic cylindrical core within a helical coil of wire causes corresponding changes in the amplitude of the voltage signal imposed across the coil. This voltage signal is then demodulated by a half-wave rectifier including a diode, a resistor and a low-pass filter. A reference signal is demodulated by a second identical half-wave rectifier. The demodulated reference signal and the demodulated coil voltage signal are fed into the inverting and non-inverting inputs of a differential amplifier having an output which drives an electrical indicating instrument.
In a similar device, such as that shown in U.S. Pat. No. 3,513,408 to McGee, the movement of a tapered magnetic core with respect to a fixed coil of wire causes changes in the inductance of the coil. These changes in inductance determine the frequency of an oscillator, and the frequency output of the oscillator is converted by a demodulator circuit into a digital signal or an analog output voltage. In order to maintain linearity, the McGee transducer depends on the use of the tapered core.
Additionally, applications exist for inductive-based measurements of an object subjected to rotary displacement. For example, there are measurements requiring knowledge regarding the magnitude of displacement to which a shaft is subjected. In one common arrangement, a potentiometer is employed to determine angular displacement of the shaft. A disadvantage inherent with this arrangement is that components of the potentiometer, such as brushes, are continually subjected to wear and tear, and thus eventually fail.